/* SPDX-License-Identifier: GPL-2.0-or-later
 * Copyright Red Hat
 * Author: David Gibson <david@gibson.dropbear.id.au>
 *
 * Tracking for logical "flows" of packets.
 */

#include <stdint.h>
#include <unistd.h>
#include <string.h>

#include "util.h"
#include "passt.h"
#include "siphash.h"
#include "inany.h"
#include "flow.h"
#include "flow_table.h"

const char *flow_type_str[] = {
	[FLOW_TYPE_NONE]	= "<none>",
	[FLOW_TCP]		= "TCP connection",
	[FLOW_TCP_SPLICE]	= "TCP connection (spliced)",
};
static_assert(ARRAY_SIZE(flow_type_str) == FLOW_NUM_TYPES,
	      "flow_type_str[] doesn't match enum flow_type");

/* Global Flow Table */
unsigned flow_first_free;
union flow flowtab[FLOW_MAX];

/* Last time the flow timers ran */
static struct timespec flow_timer_run;

/** flow_log_ - Log flow-related message
 * @f:		flow the message is related to
 * @pri:	Log priority
 * @fmt:	Format string
 * @...:	printf-arguments
 */
void flow_log_(const struct flow_common *f, int pri, const char *fmt, ...)
{
	char msg[BUFSIZ];
	va_list args;

	va_start(args, fmt);
	(void)vsnprintf(msg, sizeof(msg), fmt, args);
	va_end(args);

	logmsg(pri, "Flow %u (%s): %s", flow_idx(f), FLOW_TYPE(f), msg);
}

/**
 * DOC: Theory of Operation - allocation and freeing of flow entries
 *
 * Each flow takes a single slot in flowtab[].  Moving entries in that table
 * (which we used to do) is fiddly and possibly expensive: it requires updating
 * the hash table indexing flows, and may require updating epoll data which
 * references the flow by index.  However, we also want to keep the active
 * entries in the table compact where possible, because otherwise scanning
 * through the entire table becomes more expensive.  This describes the
 * compromise implemented below.
 *
 * Free blocks
 *    A "free block" is a contiguous sequence of unused (FLOW_TYPE_NONE) entries
 *    in flowtab.  The first entry in each block contains metadata, specifically
 *    the number of entries in the block, and the index of the next (non
 *    contiguous) free block (struct flow_free_block).
 *
 * Free block list
 *    flow_first_free gives the index of the first entry of the first (lowest
 *    index) free block.  Each free block has the index of the next free block,
 *    or MAX_FLOW if it is the last free block.  Together these form a linked
 *    list of free blocks, in strictly increasing order of index.
 *
 * Allocation
 *    When allocating a new flow, we always use the first entry of the first
 *    free block, that is, at index flow_first_free.  If the block has more than
 *    one entry, flow_first_free is updated to the next entry, which is updated
 *    to represent the new smaller free block.  Otherwise the free block is
 *    eliminated and flow_first_free is updated to the next free block.
 *
 * Scanning the table
 *    Theoretically, scanning the table requires FLOW_MAX iterations.  However,
 *    when we encounter the start of a free block, we can immediately skip to
 *    its end, meaning that in practice we only need (number of active
 *    connections) + (number of free blocks) iterations.
 *
 * Freeing
 *    We can only free entries when scanning the whole flow table in
 *    flow_defer_handler().  This is what lets us maintain the fee block list in
 *    index sorted order.  As we scan we keep track of whether the previous
 *    entry was in a free block or not.  If so when an entry is freed (its
 *    deferred handler returns 'true'), we add it to that free block.  Otherwise
 *    we create a new free block for it and chain it to the last free block we
 *    scanned.
 */

/**
 * flow_alloc_commit() - Mark a new flow as used
 * @pflow:	Unused flow returned from flow_prealloc()
 * @type:	type of flow this entry will now be used for
 *
 * Return: writable pointer to @pflow.
 */
union flow *flow_alloc_commit(const union flow *pflow, enum flow_type type)
{
	union flow *flow = (union flow *)pflow;

	ASSERT(FLOW_IDX(flow) == flow_first_free);
	ASSERT(flow->f.type == FLOW_TYPE_NONE);
	ASSERT(flow->free.n >= 1);

	if (flow->free.n > 1) {
		/* Use one entry from the block */
		union flow *next = &flowtab[++flow_first_free];

		ASSERT(FLOW_IDX(next) < FLOW_MAX);
		ASSERT(next->f.type == FLOW_TYPE_NONE);
		ASSERT(next->free.n == 0);

		next->free.n = flow->free.n - 1;
		next->free.next = flow->free.next;
	} else {
		/* Use the entire block */
		flow_first_free = flow->free.next;
	}

	flow->f.type = type;
	return flow;
}

/**
 * flow_defer_handler() - Handler for per-flow deferred and timed tasks
 * @c:		Execution context
 * @now:	Current timestamp
 */
void flow_defer_handler(const struct ctx *c, const struct timespec *now)
{
	struct flow_free_block *free_head = NULL;
	unsigned *last_next = &flow_first_free;
	bool timer = false;
	unsigned idx;

	if (timespec_diff_ms(now, &flow_timer_run) >= FLOW_TIMER_INTERVAL) {
		timer = true;
		flow_timer_run = *now;
	}

	for (idx = 0; idx < FLOW_MAX; idx++) {
		union flow *flow = &flowtab[idx];
		bool closed = false;

		if (flow->f.type == FLOW_TYPE_NONE) {
			/* Start of a free block */
			free_head = &flow->free;
			*last_next = idx;
			last_next = &free_head->next;
			/* Skip the rest of the block */
			idx += free_head->n - 1;
			continue;
		}

		
		switch (flow->f.type) {
		case FLOW_TYPE_NONE:
			closed = true;
			break;
		case FLOW_TCP:
			closed = tcp_flow_defer(flow);
			break;
		case FLOW_TCP_SPLICE:
			closed = tcp_splice_flow_defer(flow);
			if (!closed && timer)
				tcp_splice_timer(c, flow);
			break;
		default:
			/* Assume other flow types don't need any handling */
			;
		}

		if (closed) {
			memset(flow, 0, sizeof(*flow));

			if (free_head) {
				/* Add slot to current free block */
				ASSERT(idx == FLOW_IDX(free_head) + free_head->n);
				free_head->n++;
			} else {
				/* Create new free block */
				free_head = &flow->free;
				free_head->n = 1;
				*last_next = idx;
				last_next = &free_head->next;
			}
		} else {
			free_head = NULL;
		}
	}
}

/**
 * flow_init() - Initialise flow related data structures
 */
void flow_init(void)
{
	/* Initial state is a single free block containing the whole table */
	flowtab[0].free.n = FLOW_MAX;
	flowtab[0].free.next = FLOW_MAX;
}